1. Field of the Invention
The present invention relates to duplexers including first and second band-pass filters that have different pass-bands, and particularly relates to duplexers in which the first band-pass filter includes an unbalanced surface acoustic wave filter.
2. Description of the Related Art
Various duplexers that use surface acoustic wave filters have been proposed thus far. For example, Japanese Unexamined Patent Application Publication No. 2000-349591, described hereinafter, discloses a SAW duplexer including a transmission filter and a reception filter. In Japanese Unexamined Patent Application Publication No. 2000-349591, the reception filter is configured of a longitudinally coupled resonator-type surface acoustic wave filter. The reception filter configured of a longitudinally coupled resonator-type surface acoustic wave filter described in Japanese Unexamined Patent Application Publication No. 2000-349591 has an unbalanced input terminal and an unbalanced output terminal. In other words, the reception filter is an unbalanced surface acoustic wave filter.
Thus far, the stated unbalanced surface acoustic wave filter and a balanced surface acoustic wave filter that can obtain a balanced output have been known as surface acoustic wave filters used as reception filters in duplexers. The balanced surface acoustic wave filter can increase harmonic attenuation through balance between balanced output terminals.
On the other hand, in the case where an unbalanced surface acoustic wave filter such as that described in Japanese Unexamined Patent Application Publication No. 2000-349591 is used, it has been difficult to increase the harmonic attenuation. When the harmonic attenuation is low, there is a risk that a noise signal outputted from a voltage controlled oscillator (VC) will combine with the harmonic, resulting in an unnecessary signal being produced. There is thus a risk that the reception sensitivity will drop.